My iPhone was using > 2GB a month in data, and 99% of it was due to DNS. Consistently I was chewing up 50-100MB in data per day. I would go to Cellular->System Services->DNS and see very little usage elsewhere, but high GB usage for DNS.
What could possibly cause this? A DNS query is an ipv4 packet or two...
All the other data usage clearly correlated to my actual usage but I could not understand DNS.
This is on an iPhone 8+ that is always updated to the latest iOS within a few days.